Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 230830
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
430 AM EDT TUE SEP 23 2014

.SYNOPSIS...
HIGH PRESSURE WILL BUILD FROM THE NORTH THROUGH MID WEEK WHILE A
COASTAL TROUGH REMAINS OFFSHORE INTO LATE WEEK. HIGH PRESSURE
WILL REMAIN TO THE NORTH INTO THIS WEEKEND MAINTAINING PERSISTENT
NORTHEAST FLOW.

&&

.NEAR TERM /THROUGH TONIGHT/...
OVERNIGHT...THE SYNOPTIC AND MESOSCALE HAVE BEEN EXTREMELY COMPLEX
WITH THE STRONG CUT-OFF MID LEVEL LOW STALLING OVER NE GEORGIA AND
UPSTATE SOUTH CAROLINA WHILE SURFACE HIGH PRES BUILDS INTO THE MID
ATLC...WEDGING SOUTH WITH TIME AND REALLY DIGGING IN BY DAWN.
WIDESPREAD RAINS AND ELEVATED TSTMS HAVE PRODUCED SOME LOCALLY
HEAVY DOWNPOURS OVERNIGHT TO THE WEST OF I-95 UNDER A STRONG REGION
OF UPPER FORCING/FOCUSED DIVERGENCE FOR ASCENT. NEAR TERM GUIDANCE
SUGGEST THESE RAINS WILL CRAWL EAST AND DIMINISH IN INTENSITY
TOWARD DAYBREAK WHILE ADDITIONAL RAINS DEVELOP ALONG AND E OF I-95
AS A WELL DEFINED REGION OF 500-300 MB Q-G FORCING PIVOTS UNDER
THE UPPER LOW.

TODAY THE UPPER LOW IS FORECAST TO WEAKEN AND FILL WITH MID LEVEL
VORTICITY EXPECTED TO EVENTUALLY STRETCH AND SPLIT BY TONIGHT AS
THE MEAN CUT-OFF MAKES LITTLE MOVEMENT THANKS TO A BLOCKING ATLC
RIDGE OFFSHORE. THIS PATTERN WILL EVOLVE LIKE A COOL SEASON EVENT
WITH A TIGHTENING COASTAL TROUGH...ENHANCED PRES GRADIENT PINCHING
BY LATER TODAY AND TONIGHT AND UNSETTLED WEATHER WITH POCKETS OF WET
WEATHER AND PLENTY OF CLOUDS TRAPPING UNDER A DEVELOPING WEDGE
INVERSION AS UPPER LEVELS DRY OUT FROM THE SW TODAY...CLEARLY
EVIDENT ON WATER VAPOR SATELLITE IMAGERY.

THE COOL TEMPS ALOFT AND EXPECTATIONS OF STUBBORN CLOUDS AND NE FLOW
TODAY HAVE BEEN ENOUGH TO CONVINCE US TO LOWER TEMPS TODAY...MOST
ALL AREAS. HIGH TEMPS FROM THE MID 70S IN SOME SOUTH CAROLINA
ZONES TO 78-80 DEGREES FAR SW ZONES. ELEVATED INSTABILITY IS
FORECAST TO WEAKEN DURING THIS MORNING WITH NEGATIVE SI AND LI
VALUES SLOWLY BECOMING POSITIVE BY AFTERNOON. THE POTENTIAL FOR
WET WEATHER LOOKS TO HANG ON THROUGH TONIGHT WITH THE SYNOPTIC
PATTERN...DIFFICULT ON PINNING DOWN HOURLY RAIN CHANCES...MANY AREAS
LIKELY TO MEASURE AT SOME POINT TODAY...SOME EARLY..SOME LATER.

TONIGHT...CLOUDY AND COOL WEATHER WITH BREEZY TO WINDY CONDITIONS
ALONG COASTAL AREAS AS THE SURFACE TROUGH TIGHTENS JUST OFFSHORE.
SCATTERED MAINLY LIGHT RAINS ARE POSSIBLE AT TIMES. LOW IN THE UPPER
50S WELL INLAND TO LOWER 60S ALONG THE COAST.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
THERE HAS BEEN A RATHER REMARKABLE SHIFT IN THE MODELS FOR THE
MEDIUM RANGE PERIOD COMPARED TO 24 HRS PRIOR. BOTH THE GFS AND ECMWF
NOW LIFT THE UPPER LOW OUT OF THE SOUTHEAST UNITED STATES BY
WEDNESDAY NIGHT. THIS WOULD PUSH THE COASTAL TROUGH FARTHER OFFSHORE
BY THURSDAY AND FRIDAY WHILE THE SURFACE HIGH OVERSPREADS FROM THE
NORTHWEST. THE OVERALL EFFECT IS LOWER POPS ACROSS THE BOARD...
ESPECIALLY INLAND. SINCE THERE HAS BEEN SOME RUN TO RUN
INCONSISTENCY WE DID NOT COMPLETELY ERADICATE POPS DURING THIS
PERIOD. WE SHOW CHANCE POPS OVER THE WATERS AND COASTAL AREAS WITH
SLIGHT CHANCE INLAND. THUNDERSTORM POTENTIAL LOOKS TO BE CONFINED TO
THE MARINE AREAS SINCE THE COOL STABLE WEDGE OF HIGH PRESSURE WILL
MAKE CONDITIONS UNFAVORABLE FOR THUNDER ELSEWHERE. CONSIDERABLE
CLOUDINESS AND COLD ADVECTION WILL KEEP HIGH TEMPS IN THE UPPER 70S
MOST AREAS.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
CONSIDERABLE DISAGREEMENT BETWEEN MODELS IN THE LONG TERM THOUGH IT
LOOKS AS THOUGH THE EXPANSIVE HIGH WILL REMAIN NORTH OF THE AREA
THROUGH SUNDAY. ENOUGH MOISTURE AND LIFT WILL EXIST FOR ISOLATED TO
SCATTERED SHOWERS ACROSS THE AREA. POTENTIAL FOR INCREASING RAIN
CHANCES EARLY NEXT WEEK IF A FRONT LIFTS NORTH OUT OF THE GULF PER
THE 00Z ECMWF. TEMPS RETURN TO NEAR NORMAL FOR LATE SEPT BY THE
WEEKEND.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
RAINS SHOULD INCREASE IN COVERAGE OVERNIGHT AND SPREAD NORTHWARD
THROUGH THE COASTAL CORRIDOR. CIGS MAINLY VFR OVERNIGHT BUT CONFIDENCE
IN NOT GREAT GIVEN THE FACT THAT IFR CIGS WERE REPORTED ALONG
COASTAL CHARLESTON COUNTY AT 0530Z AND DATA OFFSHORE AND TO THE SOUTH
WAS LIMITED. NE WINDS WILL INCREASE TODAY AND THIS EVENING AS A WEDGE
PATTERN EVOLVES. WE HAVE INTRODUCED GUSTS NEAR 20 KT AT TIMES AT
BOTH KCHS AND KSAV THIS AFTERNOON AND EVENING. RAINS ARE LIKELY THIS
MORNING WITH CHANCES THIS AFTERNOON AND EVENING. CIGS ARE QUITE
TRICKY AND MANY MODELS SUGGEST MVFR OR LOWER CIGS ARE POSSIBLE LATER
TODAY INTO TONIGHT UNDER A STRONG WEDGE INVERSION.

EXTENDED AVIATION OUTLOOK...MVFR OR LOWER CIGS POSSIBLE AT TIMES
INTO THE END OF THE WEEK WITH A LINGERING WEDGE PATTERN AND AMPLE
LOW LEVEL MOISTURE.

&&

.MARINE...
STRONG COOL SEASON LIKE WEDGING ANTICIPATED FOR THE WATERS WITH
A VERY COMPLEX INITIALIZATION EARLY THIS MORNING WITH STRONG
FRONTAL FEATURES...NUMEROUS BOUNDARIES AND A FEW SUPERCELL TSTMS
PROBABLY PRODUCING WATERSPOUTS OVERNIGHT.

THE FOCUS OF THE FORECAST SHIFTS TO A PINCHING PRES GRADIENT AS
HIGH PRES BUILDS IN FROM THE N TODAY. WITH CHAOTIC CONDITIONS OVER
THE WATERS AT 4 AM...WE INITIALIZED WITH ADVISORIES FOR WATERS
EXCEPT THE CHARLESTON HARBOR WHICH WILL SEE DETERIORATION CONDITIONS
THIS AFTERNOON. TONIGHT...A SOLID SCA ALL WATERS AS SEAS BUILD TO
6 TO 8 FT...HIGHEST OFFSHORE. THE NAM SUGGESTS WE COULD SEE SOME
LOCAL PINCHING AND A FEW GUSTS NEARING GALES AT TIMES BUT WE CAPPED
ALL WINDS AT 30 KT FOR NOW. WAVEWATCH GUIDANCE LOOKED UNDERDONE
AND WE BUMPED SEAS UP TO 6 TO 8 FT BY LATER TONIGHT...HIGHEST
OFFSHORE.

HIGH PRESSURE TO THE NORTH WITH A PROLONGED NORTHEAST GRADIENT
EXPECTED OVER THE WATERS DURING THE PERIOD. THE BEST SURGE WILL
PERSIST FROM TUESDAY THROUGH WEDNESDAY EVENING WITH SMALL CRAFT
ADVISORIES FOR ALL WATERS. LATE WEEK INTO THE WEEKEND THE GRADIENT
LOOKS TO WEAKEN SOMEWHAT...HOWEVER WE COULD AT TIMES FLIRT WITH
ADVISORY CRITERIA.

RIP CURRENTS...LOCAL GUIDANCE YIELDED A MODERATE RISK FOR RIP
CURRENTS ON AREA BEACHES TODAY...MAINLY DUE TO STRONG ONSHORE
WINDS. AN ELEVATED RISK FOR RIP CURRENTS WILL CONTINUE THROUGH AT
LEAST LATE THIS WEEK...AND PERHAPS EVEN INTO THE WEEKEND.

&&

.TIDES/COASTAL FLOODING...
A PROLONGED NORTHEAST FLOW WITH MODERATE WIND SPEEDS WILL RESULT IN
ABOVE PREDICTED TIDE LEVELS THROUGH THE UPCOMING WEEKEND. ALTHOUGH
THE POSITIVE ANOMALY WOULD NEED TO BE A LITTLE MORE THAN 1.0 FEET
THERE IS A MODERATE CHANCE OF THE TIDE REACHING 7 FT MLLW ALONG THE
SC COAST WHICH WOULD NECESSITATE COASTAL FLOOD ADVISORIES DURING THE
EVENING HIGH TIDES.

&&

.EQUIPMENT...
THE DOWNTOWN CHARLESTON WEATHER SENSOR AT WATERFRONT PARK /KCXM/
IS INOPERABLE. IT IS UNKNOWN AT THIS TIME WHEN THE SENSOR WILL BE
RETURNED TO SERVICE. HOURLY OBSERVATIONS AND DAILY CLIMATE
INFORMATION WILL NOT BE AVAILABLE UNTIL FURTHER NOTICE.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 2 PM THIS AFTERNOON TO 11 AM EDT
     WEDNESDAY FOR AMZ330.
     SMALL CRAFT ADVISORY UNTIL MIDNIGHT EDT WEDNESDAY NIGHT FOR
     AMZ350-352-354-374.

&&

$$

NEAR TERM...
SHORT TERM...JRL
LONG TERM...JRL
AVIATION...
MARINE...JRL
TIDES/COASTAL FLOODING...
EQUIPMENT...





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